The present invention relates to a stainless steel, in particular to a stainless steel having excellent antibacterial property and being suitable for the applications of, for example, kitchen utensils and other daily utensils, medical devices, electrical equipment, chemical instruments and construction materials. The steels in the present invention include steel sheets, steel strips, steel pipes and steel wires.
Silver and copper have been known to have effects of suppressing growth of pathogenic bacteria typically including Escherichia coli and salmonellae and hence preventing food poisoning linked to such pathogenic bacteria.
Recently, materials obtained by using these metals and having inhibitory effect on bacterial growth (hereinafter referred to as xe2x80x9cantibacterial propertyxe2x80x9d) have been proposed.
By way of illustration, Japanese Unexamined Patent Publication No. 8-49085 discloses a stainless steel sheet having excellent antibacterial property obtained by forming a metal layer or alloy later of Cr, Ti, Ni, Fe or the like containing Ag and/or Cu on the surface of a stainless steel matrix through magnet sputtering. This steel sheet is preferably obtained by forming a metal layer or alloy layer containing 19 to 60 wt % of Ag.
Separately, Japanese Unexamined Patent Publication No. 8-156175 proposes a coated steel sheet obtained by applying a pigment containing silver to suppress bacterial growth.
However, the aforementioned process of forming a metal layer or alloy layer containing an antibacterial metal onto the surface of a steel sheet and the process of applying a pigment containing an antibacterial metal have the following problems: The surface layer containing the antibacterial metal is peeled or removed through drawing or grinding of the surface, and the effects of the surface layer are no longer provided. In addition, the antibacterial property cannot be retained for a long duration in the applications where the surface of the steel is always rubbing such as in a steel sheet used for interior trim of washing machines or in the applications where the surface of steel is always rubbed by cleansing as in kitchen utensils. According to the above processes, extra manufacturing steps for coating or for forming a metal or alloy layer are required than conventional processes, and with a decreasing thickness of sheet the surface area per unit weight increases and hence the coating amount or the amount of the metal layer or alloy layer per unit weight increases, which results in unfavorably increasing costs.
Japanese Unexamined Patent Publication No. 8-239726 discloses an antibacterial and anti-maricolous-organism material comprising, by weight, Fe: 10 to 80%, Al: 1 to 10%, or in addition, 1 to 15% of at least one member of Cr, Ni, Mn, Ag with the balance being copper and incidental impurities. This material is, however, a copper-based alloy or iron-based alloy containing 1 to 10% Al, has low workability and is still problematic for the application as thin steel sheets as in eating utensils, kitchen utensils and parts of electrical equipment.
To solve the aforementioned problems, Japanese Unexamined Patent Publication No. 8-104953 proposes an austenitic stainless steel having improved antibacterial property obtained by adding 1.1 to 3.5 wt % Cu, and Japanese Unexamined Patent Publication No. 8-104952 proposes a martensitic stainless steel having improved antibacterial property obtained by adding 0.3 to 5 wt % Cu.
According to the technologies described in Japanese Unexamined Patent Publication No. 8-104953 and Japanese Unexamined Patent Publication No. 8-104952, however, Cu as ions must be eluted from the surface of the steel sheet to develop antibacterial property. The elution of Cu as ions means the destruction of a passivation film at the same site, and hence the corrosion resistance is extremely deteriorated although the antibacterial property is improved. According to such a Cu-added stainless steel, therefore, the antibacterial property can hardly be compatible with the corrosion resistance.
It is an object of the present invention to provide both a stainless steel and a method of producing the same, which stainless steel can advantageously solve the problems of conventional technologies and has excellent workability and corrosion resistance, and in addition has still excellent antibacterial property even when subjected to currently-employed surface finishing inclusive of grinding.
The present inventors made intensive investigations on the relation between the chemical composition of the surface of a stainless steel sheet and the antibacterial property, using analyzers such as a field emission type Auger electron spectroscope and an electron beam microanalyzer in order to develop a stainless steel sheet compatibly having antibacterial property, and excellent workability and corrosion resistance. Consequently, they newly found that stainless steel sheets having high antibacterial property and, add to this, excellent workability and corrosion resistance can be obtained by adding a proper amount of Ag to a stainless steel and making one or more members of silver particles, silver oxides and silver sulphides to occur on the surface of resultant stainless steel sheets. They further found that these stainless steel sheets have stable antibacterial property even in the applications to be subjected to forming or grinding or in the applications where the surfaces are rubbed or abraded.
The present invention has been accomplished based upon the above findings and further investigations.
(1) A stainless steel having excellent antibacterial property and containing 10 wt % or more Cr and 0.0001 to 1 wt % Ag, wherein the steel includes a total of 0.001 or more in area percentage of one or more members selected from a silver particle, a silver oxide and a silver sulphide.
(2) The stainless steel having excellent antibacterial property according to (1), wherein the stainless steel contains one or more members selected from: Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt %.
(3) The stainless steel having excellent antibacterial property according to (1) or (2), wherein the silver particle, silver oxide and silver sulphide each have a mean grain diameter of 500 xcexcm or less.
(4) A method of producing a stainless steel material having excellent antibacterial property, which method comprises continuously casting a molten stainless steel containing Cr: 10 wt % or more, Ag: 0.0001 to 1 wt % to give a steel material, wherein the casting rate of the continuous casting ranges from 0.8 to 1.6 m/min.
(5) The method of producing a stainless steel material having excellent antibacterial property according to (4), wherein the molten stainless steel contains one or more members selected from Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt %.
(6) A method of producing a cold-rolled stainless sheet steel, wherein the stainless steel obtained according to (4) or (5) is further subjected to hot-rolling, cold-rolling.
The content limits of the chemical composition of the steel according to the present invention will now be described.
The steel of the invention can advantageously be applied to any of austenitic stainless steels, ferritic stainless steels, martensitic stainless steels and a variety of other stainless steels.
The austenitic stainless steel preferably has a chemical composition of: C: 0.01 to 0.1 wt %, Si: 2.0 wt % or less, Mn: 2.0 wt % or less, P: 0.08 wt % or less, S: 0.02 wt % or less, Cr: 10 to 35 wt %, Ni: 6 to 15 wt %, N:0.01 to 0.1 wt % with the balance being Fe and incidental impurities. The steel may further comprise one or more members selected from: Mo: 3.0 wt % or less, Cu: 1.0 wt % or less, W: 0.30 wt % or less, V: 0.30 wt % or less, Al: 0.3 wt % or less, Ti: 1.0 wt % or less, Nb: 1.0 wt % or less, Zr: 1.0 wt % or less, B: 0.01 wt % or less.
The ferritic stainless steel preferably has a chemical composition of: C: 0.01 wt % or less, Si: 1.0 wt % or less,Mn: 2.0 wt % or less, P: 0.08 wt % or less, S: 0.02 wt % or less, Cr: 10 to 35 wt %, N: 0.10 wt % or less with the balance being Fe and incidental impurities. The steel may further comprise one or more members selected from: Al: 0.3 wt % or less, Ni: 1.0 wt % or less, Mo: 3.0 wt % or less, Ti: 1.0 wt % or less, Nb: 1.0 wt % or less, V: 0.30 wt % or less, Zr: 1.0 wt % or less, Cu: 1.0 wt % or less, W: 0.30 wt % or less, B: 0.01 wt % or less.
The martensitic stainless steel preferably has a chemical composition of: C: 0.01 to 0.07 wt %, Si: 1.0 wt % or less, Mn: 2.0 wt % or less, P: 0.08 wt % or less, S: 0.02 wt % or less, Cr: 12 to 17 wt %, N: 0.007 to 0.03 wt % with the balance being Fe and incidental impurities. The steel may further comprise one or more members selected from: Al: 1.5 wt % or less, Ti: 0.6 wt % or less, Nb: 0.5 wt % or less, V: 0.30 wt % or less, W: 0.30 wt % or less, Zr: 1.0 wt % or less, Ni: 3.0 wt % or less, Mo: 3.0 wt % or less, Cu: 1.0 wt % or less, B: 0.01 wt % or less.
According to the present invention, Ag: 0.0001 to 1 wt %, or, in addition, one or more members selected from Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt % are added to a stainless steel, preferably to a stainless steel having the chemical composition of the aforementioned range.
Cr: 10 wt % or more.
Cr is an essential alloy component to ensure corrosion resistance of the stainless steels and is required to be contained in a content of 10 wt % or more.
Ag: 0.0001 to 1 wt %.
Ag is the most important element in the present invention and is an element acting to suppress bacterial growth and to enhance antibacterial property. Ag provides these benefits when at least 0.0001 wt % is present. On the other hand, if Ag content exceeds 1 wt %, the corrosion resistance is deteriorated though the antibacterial property is enhanced, and surface defects are increased in a hot-rolling process. In addition, a large amount of expensive Ag must be added, thereby increasing costs. Consequently, Ag content is controlled to the range of 0.0001 to 1 wt %. Ag content is more preferably less than 0.05 wt %.
According to the present invention, Ag to be contained in the steel should be contained as one or more members selected from an Ag (silver) particle, a silver oxide and a silver sulphide in total in an area percentage of 0.001% or more. Ag as an Ag (silver) particle, silver oxide or silver sulphide which is dispersedly present on the surface of a steel material in use suppresses bacterial growth and markedly enhances antibacterial property. The Ag (silver) particle, silver oxide and silver sulphide may be present independently or as a complex compound composed of two or three members.
The persistent presence of the silver particle, silveroxide or silver sulphide dispersedly on the surface of the steel in use is essential to ensure stable antibacterial property. The silver particles, silver oxides or silver sulphides are preferably present on the surface, not only on the surface upon shipment of steel products but also on the surface after polishing, cutting/grinding or the surface of steel in use where a new surface is formed by abrasion or the like.
The presence of Ag in the steel is evaluated by the area percentage in the surface of a cross section to be determined, which area percentage is measured by subjecting an arbitrary cross section of a test piece sampled from the steel to element mapping determination with an X-ray microanalyzer.
When the total content of one or more members selected from a silver particle, a silver oxide and a silver sulphide is less than 0.001% in area percentage, no suppressing effect on bacterial growth is observed and no antibacterial property is exhibited. On the other hand, if the total content in area percentage exceeds 30%, the benefits of enhancing antibacterial property no more accrues and Ag content increases, thereby increasing costs, and, in addition, deteriorating corrosion resistance. Consequently,the total content of one or more members selected from a silver particle, a silver oxide and a silver sulphide is controlled to the range from 0.001% to 30% in area percentage. The mean grain diameters of the silver particle, silver oxide and silver sulphide exceeding 500 xcexcm can cause deterioration of corrosion resistance and workability. Therefore, the components preferably have a mean grain diameter of 500 xcexcm or less.
According to the present invention, it is desirable that the steel further comprises one or more members selected from: Sn: 0.0002 to 0.02 wt %, Zn: 0.0002 to 0.02 wt %, Pt: 0.0002 to 0.01 wt %, in addition to Ag in the above range.
Each of Sn, Zn, Pt acts to disperse and precipitate the silver particle, silver oxide, silver sulphide and to thereby stabilize the development of antibacterial property. At least 0.0002 wt % for Sn, at least 0.0002 wt % for Zn and at least 0.0002 wt % for Pt must be present to obtain these benefits. On the other hand, if the contents exceed 0.02 wt % for Sn, 0.02 wt % for Zn and 0.01 wt % for Pt, the benefits do no more accrue, and workability and corrosion resistance are liable to be deteriorated. The contents are, therefore, preferably controlled to the ranges of 0.0002 to 0.02 wt % for Sn, 0.0002 to 0.02 wt % for Zn and 0.0002 to 0.01 wt % for Pt.
The stainless steel of the present invention is composed of, in addition to the above chemical composition, the balance being Fe and incidental impurities. From the viewpoint of preventing the deterioration of corrosion resistance, the content of soluble oxides and sulphides other than silver oxides and silver sulphides is preferably reduced as much as possible.
The steel of the present invention can be formed into an ingot by applying any of conventional known steel making techniques and hence the steel making technique used in the invention is not limited. Regarding steel making techniques, the molten steel can be prepared by, for example, refining in a converter or an electric furnace and then to secondary refining by SS-VOD (Strongly Stirred Vacuum Oxygen Decarburization).
The molten steel obtained by steel making technique can be formed into a steel material by any of conventional known casting methods, whereas continuous casting is preferably employed for productivity and quality.
In the continuous casting, the casting rate preferably ranges from 0.8 to 1.6 m/min in order to disperse the silver particle, silver oxide, silver sulphide in the steel finely and uniformly.
When the casting rate is less than 0.8 m/min, the silver particle, silver oxide or silver sulphide becomes coarse, thereby deteriorating corrosion resistance and inhibiting stable development of antibacterial property. On the other hand, when the casting rate exceeds 1.6 m/min, Ag is not uniformly dispersed in the steel, and hence the silver particle, silver oxide or silver sulphide is not dispersedly present on the surface of the steel in use, thereby inhibiting stable development of antibacterial property. For these and other reasons, the casting rate in the continuous casting preferably ranges from 0.8 to 1.6 m/min.
According to the present invention, a molten stainless steel having the above chemical composition is subjected to,preferably continuous casting under the above conditions, to give a steel material, and subsequently the steel material is heated to a given temperature according to necessity and hot-rolled under generally known hot-rolling conditions to give a hot-rolled steel sheet having a desired thickness. The hot-rolled steel sheet is annealed at 700 to 1180xc2x0 C. according to the steel composition and then cold-rolled under general known cold-rolling conditions to give a cold-rolled steel sheet having a given thickness.
The cold-rolled steel sheet is preferably subjected to annealing at 700 to 1180xc2x0 C. according to the steel composition and pickling to give a cold-rolled and annealed steel sheet.